1. Field of the Invention
This invention relates to a circuit breaker, and, more particularly, to a shield structure which interlocks with a line side of the circuit breaker and which carries and directs the electrical cables and the ionized gases away from the circuit breaker.
2. Description of the Prior Art
The demands of the electrical industry are for smaller electrical power components, which include a circuit breaker, particularly a circuit breaker used as a motor circuit protector (MCP) or an overcurrent protective device of a motor controller. When a circuit breaker is reduced in physical size, but not in electrical performance and capacity, several problems arise.
For instance, for a circuit breaker previously having a frame size of about 4 inches wide and 6 inches high, and a rated current of 63 amps, a specific installation may require the frame size to be reduced to about 3 inches wide and 4.75 inches high for the same current value. This latter frame size, in turn, requires a smaller case size for the circuit breaker, and the smaller case size results in a smaller arc chamber compared to that of the previous circuit breaker frame size of 4 inches wide and 6 inches high.
When a current interruption occurs in this smaller arc chamber, the volume of the ionized gases formed by the electrical arc created between the electrical contacts upon separation is not reduced, but is the same volume as that formed in the larger circuit breaker frame size. In the reduced frame size, this volume of gases is forced to be vented through reduced openings in the molded case, which may result in an increase in gas pressure. If the gas pressure is increased, care must be taken so that the line terminals do not restrike phase to phase since it would be the tendency for the ionized gases to be trapped in the top of the circuit breaker on its line side. That is, when a certain type of circuit breaker is a motor circuit protector or an over-current protector, installed in a motor control center where a steel barrier member divides a lower combination motor controller from the one directly above it, the steel barrier member tends to trap the ionized gases on the top of the lower circuit breaker on its line side, thereby allowing the line terminals to restrike.
One way in which to prevent the ionized gases from being trapped between the steel barrier member and the top of the circuit breaker on its line side, is to increase the space between the top of the circuit breaker and the steel barrier. However, this solution may be unacceptable since it goes in a direction opposite to miniaturization of the electrical power components, and the assemblies which they fit into.
There remains, therefore, a need in the art to decrease the size of a circuit breaker and lessening or minimizing the pressure of the ionized gases by reducing the volume of the ionized gases in the line side of the circuit breaker.
There also remains a need in the art to decrease the size of a circuit breaker without increasing the likelihood of a secondary are being struck between the line terminals as a result of the higher gas pressure created by a current interruption.